Ambient mass spectrometry: Airborne contaminants

Ezine

Published: Oct 1, 2017

Author: Steve Down

Channels: Base Peak

Ambient mass spectra

Since ambient mass spectrometry first hit the headlines with the announcement of desorption electrospray ionisation (DESI) and Direct Analysis in Real Time (DART), around 50 different techniques have been developed in the following 12 years or so. These simplified methods for analysing solids, liquids and gases require minimal sample preparation, and in some cases, zero sample preparation, because ionisation takes place in the open air directly from the samples.

When ion formation is carried out directly in front of the inlet to a mass spectrometer, the ions that are formed survive sufficiently long to be drawn into the instrument for analysis. The overall process is gentle, so that the protonated and deprotonated molecules that form tend to be the dominant ions. Their detection in the mass spectrometer is often all that is required for analyte identification but they can be fragmented in tandem techniques, if necessary, to provide structural information.

For many of the ambient methods, the ionisation process generates charged clusters of water molecules from water vapour in the air, which transfer their charge to the analytes from the sample. However, other gaseous species like ammonia can also be ionised. Even though ammonia is present at far lower concentrations than water vapour, its higher proton affinity ensures that ammonium ions are formed. These can form adducts with the analytes to produce further peaks in the spectra, although they can be useful for confirming molecular masses.

Three ambient systems compared

The most common way of dealing with mass spectral peaks originating from airborne species in ambient mass spectra is to subtract the background spectrum from the sample spectrum but this assumes that the contaminants are present in steady concentrations. That is not always the case. The levels of numerous contaminants arising from normal lab activities, such as solvents and chemicals from cleaning products and personal care products, vary over time.

It is compounds like this that have attracted the attention of a team of scientists at the University of California, Irvine. Barbara Finlayson-Pitts, Sambhav Kumbhani, Lisa Wingen and Véronique Perraud thought that the contaminants might influence ambient mass spectra and interfere with compound identification and measurement. As a first test, they have examined the effects of chemicals derived from floor treatment products that were routinely applied in their building.

In order to compensate to some extent for the large number of ambient ionisation techniques that are available, the experiments were replicated on three systems. For extractive electrospray ionisation (EESI), a charged solvent spray is crossed with an analyte spray. In DART, metastable ions in a gas plasma produced by electrical discharge react with atmospheric water molecules to produce charged water clusters that ionise the analytes.

The third ambient technique uses the lesser known piezoelectric direct discharge (PDD) in which compression of a piezoelectric crystal in ambient air produces a low current and a high potential that generates ions which, in turn, ionise the analytes in the sample. All three ionisation methods tend to produce protonated molecules in positive ion mode, with DART also producing ammonium ion adducts.

Indoor air contaminants

The experiments did not have any analytical samples in the true sense. Instead, they analysed the ambient laboratory air before and after treatment of one floor with waxing and stripping solutions. The mass spectra from all three systems contained intense peaks at m/z 135 and 152 after treatment that were absent beforehand.

The DART and PDD instruments were in a different lab which was not treated yet the same peaks were observed in all cases, suggesting widespread airborne contamination. After the floor was rinsed with water and allowed to dry, the intensities of the two peaks fell by an order of magnitude, supporting the floor treatment as the source.

A sample of the waxing solution was analysed by EESI mass spectrometry to try and identify the contaminants. After comparison with spectra of standard compounds, m/z 135 and 152 were attributed to the [M+H]+ and [M+NH4]+ ions of diethylene glycol monoethyl ether (DEGMEE) which is a major ingredient of the wax. This was confirmed by tandem mass spectrometry. The ammoniated peak was more intense than the protonated peak due to the double presence of ammonia in the air and the waxing solution itself.

A second pair of much smaller peaks at m/z 119 and 136 were attributed to the protonated and ammoniated forms of ethylene glycol monobutyl ether. A number of different glycol ethers are added to cleaning products, so the data are consistent with the expected composition.

These ether contaminants had two major effects on the mass spectra of ambient air. They induced major ion suppression, as illustrated for atomised pentanedioic acid particles, reducing their intensity by more than 75% while introducing large glycol ether peaks. In addition, they produced peaks for a butylamine adduct of DEGMEE.

These experiments illustrate that routine background extraction might not be adequate for ambient mass spectrometry and more care should be taken to identify any local airborne interfering compounds. Different contaminants will dominate in different labs. However, the research team make a point of concluding "this drawback of ambient air contaminants is substantially outweighed by the versatility and simplicity of ambient ionization mass spectrometry techniques."